When users "share desks" then only one person has the extension of the actual phone, the others must be adjuncts, or "phantom" extensions. Then as TTT says, they can use the follow me feature to be "logged in anywhere. There will be an issue with voicemail message waiting indicators, but they will just have to work that out.
Caution: if you are in a competitive sales scenario, this could be a disaster, as someone would quickly figure out they could answer $ale$ calls from many agents from their one phone. You might want to have them use the forward feature on an adjunct, and give each of them an authorization code, so that code would have to be known in order to forward the calls.
Pepperzgm at gmail dotte comm